The technology of the burning glass or fire-starting lens has been known since antiquity. Fire-starting lenses are still used to light fires in outdoor and primitive settings. A fire-starting lens is simply a small powerful magnifying glass. Any sort of magnifying lens may be used to start a fire on a sunny day. The lens is held at such an angle as to focus the sun's light into as small an area as possible. By placing some tinder under this spot the tinder will soon start to smoke and catch fire.
Campers, hikers, and the like use fire-starting lenses to start fires outdoors for cooking, to sterilize water, for warmth, to light up the darkness, and for some degree of protection against animals. They also carry canteens for water. It is known to have one side of a canteen as a lens to heat water within the interior of the canteen, as disclosed in U.S. Pat. No. 4,696,284. There are two chambers in the canteen of this patent, with one being for water and the other for air. The air chamber is positioned between water chamber and the lens. The lens focuses sunlight on the water chamber to heat the water therein. The lens as it is constructed in this canteen is not useful as a fire-starting lens. What would be useful, but heretofore unavailable, is a lens in a canteen that can be used as a fire-starting lens to start fires.